drm/xen-front: Make structure fb_funcs constant

drm/xen-front: Make structure fb_funcs constant

Static structure fb_funcs, of type drm_framebuffer_funcs, is used only
when it is passed to drm_gem_fb_create_with_funcs() as its last
argument. drm_gem_fb_create_with_funcs does not modify its lst argument
(fb_funcs) and hence fb_funcs is never modified. Therefore make fb_funcs
constant to protect it from further modification.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190813062712.24993-1-nishkadg.linux@gmail.com